Reland of Fix async/await memory leak (patchset #1 id:1 of https://codereview.chromium.org/2348403003/ )

This patch closes a memory leak in async/await where the desugaring
was creating a situation analagous to that described in v8:5002.
Intermediate Promises were being kept alive, so a long-running loop
would cause linear memory usage on the heap. This patch returns
undefined to the 'then' callback passed into PerformPromiseThen
in order to avoid this hazard. Test expectations are fixed to remove
expecting extraneous events which occurred on Promises that are
now not given unnecessarily complex resolution paths before being
thrown away.

This patch is a reland; originally, tests which exercised the memory
exhaustion were checked in. Although it's possible to find good parameters
for running such tests locally, it is difficult to automate the tests
between the rock of timeouts and the hard place of too-small heaps
causing memory exhaustion in some modes even when there is no leak.

BUG=v8:5390

Review-Url: https://codereview.chromium.org/2352933002
Cr-Commit-Position: refs/heads/master@{#39520}